Running through some of Windows Phone 7's first games and apps, it's hard not to be impressed. Twitter looks and feels fantastic, and you have to check out this ridiculous 3D tower defense game, Bye Bye Brain. Update: More apps!
So far, games are the real standout, particularly the titles developed by Microsoft Game Studios—they're seriously polished, and show the huge potential of Windows Phone as a gaming platform. With Xbox Live, it'd be dumb for Microsoft not to make gaming a huge part of their push. Windows Phone is the portable Xbox.
The other apps look pretty great, particular for being right out the gate. That's in large part thanks to the default Windows Phone 7 UI, which gives them a solid template to work from, so nothing's super gross and cluttered. Like AT&T's U-Verse app very easily could've been: